This invention relates to a process for the production of the polyester poly(trimethylene terephthalate) (PTT) from 1,3-propanediol (PDO) and an aromatic acid or a dialkyl ester thereof by melt polymerization followed by solid state polymerization (SSP). More particularly, this invention relates to an improved solid state polymerization step in the production of poly(trimethylene terephthalate).
Poly(trimethylene terephthalate) is a new polyester with a unique combination of properties that are particularly suitable for carpet and textile fiber applications The molecular weight of poly(trimethylene terephthalate) required for fiber applications is in between 18,000 and 22,000 which is equivalent to an intrinsic viscosity (IV) between 0.80 and 0.94 dl/g as measured in a 60/40 phenol/tetrachloroethane solvent at 30xc2x0 C. Up to the present time, it has been found to be very difficult and very expensive to produce PTT with such an IV and good color by melt phase polymerization alone. Thus, it is necessary to use a combined melt/solid state polymerization process to produce high quality PTT for fiber applications.
First a PTT prepolymer with an intermediate IV is produced by melt polymerization. The prepolymer pellets thus produced are further polymerized in solid state to the desired IV. In the case of the much more well known poly(ethylene terephthalate) (PET), a resin with a molecular weight of 20,000 (or an IV of 0.63 dl/g) for textile fiber applications can be produced by melt polymerization alone. However, solid state polymerization has been widely used to produce PET resins with IV""s higher than 0.70 dl/g for bottle, food tray, and tire cord applications. Two different solid state polymerization processes have been practiced in the production of high IV PET, continuous and batch processes. The continuous process typically uses a cylindrical reactor wherein polycondensation takes place as the precrystallized and preheated PET pellets move downward from the top to the bottom by gravity while a stream of nitrogen flows upward countercurrently to the polymer flow to sweep away the reaction byproducts. The batch process typically uses a tumbling reactor wherein polymerization takes place under a vacuum. In PET manufacture, it is desirable to conduct solid state polymerization at or close to the maximum safe temperature since the polymerization rate increases with increasing reaction temperature. However, as the temperature increases, the tendency of polymer pellets to agglomerate or stick increases.
Unmodified PET widely used for fiber and food tray applications has a normal DSC melting point (Tm) of about 260xc2x0 C. and a glass transition temperature (Tg) of about 74xc2x0 C. The normal melting point is the peak temperature of the fusion endotherm on the DSC thermogram of the amorphous polymer sample which has zero or minimal crystallinity. This polymer can be solid state polymerized in a continuous moving bed reactor at a maximum average temperature of about 220xc2x0 C. without sticking or lumping of polymer pellets. This maximum allowable solid state polymerization temperature is about 40xc2x0 C. below the normal melting point. In a batch process, the maximum safe temperature is 10 to 15 degrees higher because of the tumbling action of the batch reactor. Modified PET widely used for soft drink bottle applications has a normal melting point between 250 and 255xc2x0 C. and a Tg of about 74xc2x0 C.
Since the continuous PET solid state polymerization processes are practically the only existing commercial scale solid state processes for polyesters today, according to the conventional wisdom of experts in the field, they are naturally used as the basis for the development and design of solid state polymerization processes for new polyesters. PTT has a normal DSC melting point of about 228xc2x0 C. and a Tg of about 45xc2x0 C. According to the above described conventional wisdom in the art, it is generally believed that the maximum allowable solid state polymerization temperature for the solid state polymerization of a polyester is not higher than 35 to 40xc2x0 C. below its normal melting point. Thus, the conventional wisdom would lead one to the conclusion that the maximum allowable safe reaction temperature for continuous solid state polymerization of PTT would not be higher than about 190xc2x0 C. for the continuous solid state process and about 207xc2x0 C. for the batch process. The disadvantage of theses temperatures is that the polymerization rate is relatively slow.
In the present commercial practice, the standard pellet size for PTT melt resins which are introduced into the solid state polymerization step is about 2.5 g/100. This was also the standard pellet size for some producers of PET bottle resins for many years. Other producers of PET bottle resins use pellet sizes between 1.8 and 3.0 g/100.
The present invention provides a solid state polymerization process which allows a significant rate increase over the current commercial practice and what is expected according to the conventional wisdom in the art.
The present invention is a process for producing poly(trimethylene terephthalate) (PTT) at an increased solid state polymerization rate, wherein 1,3-propanediol and an aromatic acid or a dialkyl ester thereof are esterified or transesterified, the esterification or transesterification product is polycondensed, and the polycondensation product is pelletized and then subjected to continuous solid state polymerization at a temperature of 200 to 225xc2x0 C., preferably 210 to 225xc2x0 C. In a second embodiment, the solid state polymerization is carried out in batch fashion at a temperature of 210 to 225xc2x0 C. In another embodiment of the present invention, the temperature range is 200 to 220xc2x0 C. for a continuous process and 210 to 220xc2x0 C. if it is carried out in batch, and the pellet size of the polymer to be solid state polymerized is from 1.0 to 1.5 g/100. In still another embodiment of the present invention, the solid state polymerization, batch or continuous, is carried out at 210 to 225xc2x0 C. for from 1 to 20 hours and then the temperature is increased to above the normal melting point of the prepolymer to produce a PTT polymer with an intrinsic viscosity of 1.3 dl/g or higher.